A problem basic to all rock drills either of the rotary auger type or the rotary percussive type is the variable hardness of the materials being drilled. This variation requires that auger or percussion drills have adjustable energy inputs for substantially constant feed rates. It also makes the ability to quickly switch from a rotary auger drill to a rotary percussive drill very desirable.
Schroeder U.S. Pat. No. 2,804,751 discloses a hydraulic circuit for a rotary auger coal drill having automatic feed control based upon resistance to rotation. U.S. Pat. No. 3,979,944 discloses a hydraulic circuit for typical rotary percussion rock drill having a drill steel rotation motor, impact motor and feed motor supplied by three mutually dependent partial circuits. One pump feeds both the rotation motor and impact tool. Similar circuits are shown in U.S. Pat. Nos. 3,823,784; 3,995,700; 4,023,626; 4,246,973; and 4,356,871.